Micro-electromechanical system (MEMS) devices are used in a variety of applications such as in projection display systems or laser printers. As their name implies, these devices are very small and include a number of tiny electrically-controlled component parts. The size of MEMS devices is, of course, of great advantage because they take up very little space and consume very little power. MEMS devices are for speed and accuracy often made by automated or semi-automated processes. Note that in optical applications, such as those mentioned above, these devices are sometimes referred to as MEOMS (for micro electro-optomechanical systems). For convenience, the term MEMS will be used herein to describe both.
As might be expected, the very small components on MEMS devices are sometimes very fragile and subject to being easily damaged or degraded even in normal operation. For this reason, a protective cover is often employed even when the device is ultimately to be disposed in a relatively isolated location. This cover is designed to protect the components of the MEMS device from moisture and from deleterious materials, as well as from impact by other objects during assembly or operation. In the case of an optical MEMS device, the cover will normally be transparent, or at least clear enough to allow passage of the requisite amount of light. The cover is almost always a separate component, and must be securely mounted in such a manner so as to facilitate the function of the cover and of the device itself. As background for the present invention, an exemplary MEMS device will now be described in greater detail.
FIG. 1 is an aerial (plan) view of a portion of an exemplary MEMS device 100. In this example, the MEMS device is a DMD (digital micro-mirror device). The DMD is composed of a plurality of small, independently-movable mirrored surfaces (often referred to as micro-mirrors), numbered 124 through 129 in FIG. 1 (for convenience, the partially-shown micro-mirrors are not numbered). Each micro-mirror has a mirror via formed approximately in its center, and numbered, respectively, 130 through 135. The mirror vias are typically formed along with the mirror surface itself, and extend downwardly (into the view of FIG. 1) to connect the mirror surface to a hinge assembly disposed beneath (not shown). While only six micro-mirrors are (fully) shown in FIG. 1, a typical DMD may include on the order of thousands of them—even one million such structures or more. For convenience, the collection of micro-mirrors on MEMS device 100 will from time to time be referred to as the device's reflecting surface.
This MEMS device 100 of FIG. 1 may be part of, for example, the optical path of a projection display system. As an illustration, FIG. 2 is a simplified block diagram illustrating selected components of an optical path 210 in which the MEMS device 100 of FIG. 1 maybe employed. Briefly, light from a light source 211 is collimated and directed along a first portion 221 of the optical path 210. A color wheel 213 is used to produce selectively-colored light for creating colored images. The condenser lenses 212 and 214 shape the beam of light as it propagates along the first portion 221 of optical path 210. The selectively-colored light eventually falls on the MEMS device 100, where it is transformed into a visual image. The visual image created by MEMS device 100 is directed to a second portion 222 of the optical path 210, which includes a display screen 219. Display screen 219 presents the visual image display intended to be seen by the viewer and may be, for example, an HDTV screen. A projection lens 218 enlarges the image created by MEMS device 100 so it will fill the display screen 219.
MEMS device 100 creates these visual images by rapidly and selectively reorienting the individual mirrors formed on its reflecting surface (see FIG. 1). Each of these micro-mirrors is individually controllable to rapidly change orientation, which determines whether the mirror surface does or does not reflect light toward the second portion 222 of the optical path 210 (shown in FIG. 2). The operation of each of the micro-mirrors is governed by a controller 217 based on video information received from a video information source 216. Light not reflected toward the optical path second portion 222 may instead be directed toward a light dump (not shown) where, to avoid potential interference problems, it is absorbed rather than reflected. A protective cover may used to protect the reflecting surface of the MEMS device but, as should be apparent, any cover mounted over the micro-mirrors must be optically-suited to permit the passage of the incident and reflected light. In addition, it should be mounted in such a manner so as to provide optimal protection.
The MEMS device 100 described above, or more precisely the main operational portion of it, is herein generally referred to as the active area. The MEMS device active area is often fabricated on a thin wafer of semiconductor material along with many identical or similar devices. FIG. 3 is an aerial view of a semiconductor wafer 300 populated with a number of MEMS devices. For purposes of illustration the MEMS device 100 has been generally labeled, although it is noted that further fabrication and assembly steps are performed before the MEMS device 100 forms part of an operational system such as optical path 210 illustrated in FIG. 2. The active area 101 of MEMS device 100 is, however, substantially defined at the illustrated stage of production. Active area 101 includes, in general, the individual micro-mirrors shown in FIG. 1, as well as the hinge assembly (not shown) on which they are able to reorient, and the electronics used to induce this movement (also not shown). Wafer 300, in this example, has a flat 305 used for properly positioning the wafer 300 during fabrication.
The MEMS device active area 101 is typically formed in a series of process steps. As should be apparent, these process steps are often performed for the entire wafer at roughly the same time and may include, for example, doping certain areas of the wafer 300, or depositing and selectively etching away material in a series of layers to form the components of the various devices in the active area 101. Such devices, and those in the active areas (shown but not numbered) of other devices formed on or near the surface 304 of wafer 300 may, in this example, include the micro-mirrors themselves, the underlying hinges, and the electronic circuitry used to induce mirror movement when the MEMS device 100 is in operation. When the MEMS device active area 101 has been formed, a protective cover (not shown in FIG. 3) may be added.
When such a cover is mounted, MEMS device 100 includes a closed recess or cavity between the cover and the surface of the active area, which recess is preferably sealed. The sealing is desirable because the introduction of moisture into the MEMS cavity may cause premature device degradation. Permeation does not normally occur through the substrate of course, or through the cover, but it can occur if the adhesive bonding these components is not properly applied. (And some permeation may occur through the adhesive itself.) In this regard, uniform bondline thickness is necessary for adhesive flow control. Fillers in the shape of, for example, rods or balls may be used for this purpose. Unfortunately, these fillers may cause pressure points to form when the components are assembled, and these pressure points may lead eventually to damage to the substrate. In addition, at least some permeation is likely to occur through the adhesive itself, even if it is ideally applied. Needed then, is an improved MEMS device that minimizes these disadvantages, or avoids them altogether. The present invention provides just such a solution.